Image transducing system employing reverse
biased junction diodes

ABSTRACT

A TARGET FOR A VIDICON CAMERA TUBE COMPRISING AN NTYPE SEMICONDUCTOR MEMBER HAVING A RESISTIVITY BETWEEN 0.01 AND 0.1 OHM-CM. AND A PLURALITY OF DISCRETE JUNCTIONS ON THE SIDE OF THE TARGET MEMBER WHICH IS TO BE SCANNED BY AN ELECTRON BEAM.

1m 23, 1973 P. H. WENDLAND Re. 27,559 IMAGE TRANSDUCING SYSTEM EMPLOYING REVERSE BIASED JUNCTION DIODES Original Filed Sept. 21, 1966 5 Sheets-Sheet 1 Z/'IllnL lll I I Il ll f -IIVHI W2 Arma/M Jan. 23, 1973 P. H. WENDLAND Re. 27,559

IMAGE TRANSDUCING SYSTEM EMPLOYING REVERSE l BIASED JUNCTION DIODES Original Filed Sept. 2l, 1966 5 Shmets-Sheeil 8 Pvoroaeefwf 5447/1/5) l Jan. 23, 1973 Original Filed Sept. 2l, 1966 P. H. WENDLAND IMAGE TRANSDUCING SYSTEM EMFLOYING REVERSE BIASED JUNCTION DIODES 5 Sheets-Sheet 5 P. H. WENDLAND Re. 27,559

Jan. 23, 1973 IMAGE TRANSDUCING SYSTEM EMPLOYING REVERSE BIASED JUNCTION DIODES 5 Sheets-Sheet i Original Filed Sept.. 21, 1966 55pm/.sf 77,445 (2,:4 .925.)

P. H. wl-:NDLAND Re. 27,559 Jm 23, IMAGE TRANSDUCING SYSTEM EMPLOYING REYERSE BIASED JUNCTION DIODES Original Filed Sept. 21. 1966 5 Sheets-Sheet 5 United States Patent O 27,559 IMAGE TRANSDUCING SYSTEM EMPLOYING REVERSE BIASED JUNCTION DIODES Paul H. Wendland, Malibu, Calif., assigner to Hughes Aircraft Company, Culver City, Calif.

Original No. 3,423,623, dated Jan. 21, 1969, Ser. No. 580,962, Sept. 21, 1966. Application for reissue Apr. 8, 1970, Ser. No. 132,470

Int. Cl. Hlj 29/39, 31/26 U.S. Cl. 315- 22 Claims Matter enclosed in heavy brackets appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

ABSTRACT OF THE DISCLOSURE A target for a vidicon camera tube comprising an N- type semiconductor member having a resistivity between 0.01 and 0.1 ohm-cm. and a plurality of discrete junctions on the side of the target member which is to be scanned by an electron beam.

This invention relates generally to image transducing systems employing photosensitive, charge-storing elements and more particularly to an improved vidicon camera tube and target element for use therein.

The vidicon is a well-known television camera tube which has come into wide use since its introduction. The vidicon employs the phenomenon of photoconductivity in the target element to transduce light signals into electrical signals. The relaxation time of the photoconductor must be greater than the 1/30 of a second television raster scan time in order for the scanning readout electron beam to be able to distinguish between the illuminated and the dark areas of the target. The requirements of the vidicon target are thus basically twofold; photosensitivity with high quantum etiiciency, and a charge storage time greater than 1/30 of a second. In addition, a fast time response at all light levels is desired, and for special applications, a spectral response with high quantum efiiciency in a variety of regions such as the infrared, the visible, and/or the ultraviolet.

'I'he target materials successfully used to date for vidicon operation are compound semi-insulators with relatively large bandgaps and with resistivities above the 1011 ohm-cm. necessary to exhibit RC relaxation times greater than 1/30 of a second. Antimony trisuliide (the most common vidicon target material) exhibits an elective peak quantum eiiiciency of 7%, a spectral response from 4000 A. to 7000 A., and a time lag at low light levels. Narrower bandgap materials having more desirable characteristics (e.g. higherquantum efliciency and larger range of spectral response) than antimony trisulde for use as vidicon target materials, exhibit too low a resistivity for bulk l photoconductor vidicon operation.

Re. 27,559 Reissued Jan. 23, 1973 ICC exhibit too low a resisitivity for bulk photoconductor vidicon operation) as the target material for vidicon camera tubes.

It is a still further object o-f the invention to provide a vidicon target comprising a matrix or array of discrete diode junctions in silicon or germanium.

These objects are accomplished according to the present invention by means of a unique target by means of which charge relaxation times greater than lo of a second (a typical television scan time) are achieved in relatively low resistivity materials. This target comprises a layered structure formed of a single semiconductor material having a p+n junction between a large area n-type layer and a p-type layer formed as a mosaic array of discrete, insulatingly spaced-apart small area islands. The p-type layer faces the scanning electron beam and is charged to the cathode potential. A transparent conductive electrode in contact with the other surface of the n-type layer has applied thereto a positive potential to reverse bias the p+n junction. Such a target constructed of silicon having a resistivity between 0.01 and 0.1 ohm-cm. exhibits a charge relaxation time greater than the lf3@ of a second television scan time. Any photosensitive semiconductors having a resisitivity satisfying the following formula can be used:

wherein By means of this design the performance and improvements specified above are achieved. For example, reverse biased p+n junctions in silicon have demonstrated peak quantum efficiencies for photoconductivity of 35%, a spectral response from 3500 A. to 11,000 A., and a response time of microseconds. It was not known, prior to the present invention, that a target structure could exist which could satisfy the requirements for vidicon charge storage in relatively low resistivity semiconductors. Advantageously, even high resistivity semiconductors like antimony trisultide can be used since it is possible by proper doping to obtain low resistivity from high resistivity materials.

Previous vidicon camera tubes were too insensitive and too slow at low light levels to be used extensively in commercial television. A vidicon camera tube made according to the present invention, however, provides performance equal to that of the image orthicon camera tube while providing lower price and longer service. The present invention can also be used as an image transducing device having applications in the infrared. The target ofthe present invention, with a response ont to 1.1 microns, can fuliill the need for an image transducing device useful at night, since it is noted that a substantial portion of the illumination of the night sky is located at 1.0 micron.

These and other objects and advantages of the present invention will be more fully understood by reference to he following detailed description when read in conjuncion with the attached drawings wherein like reference lumerals indicate like elements and in which:

FIG. 1 is a cross-sectional, partly schematic view hrough a vidicon camera tube of the present invention,

FIG. 2 is a front View of the target of FIG. 1 and :hows the mosaic array of p-n junctions, according to :he invention,

FIG. 3 is a schematic diagram of an equivalent circuit )f a reverse biased junction diode,

FIGS. 4A and 4B are graphs showing the charge decay 1nd voltage decay, respectively, of a reverse biased junc- :ion diode,

FIG. 5 plots dark charge storage time vs. base re- :istivity with reverse leakage current as a parameter,

FIGS. 6A, 6B and 6C schematically illustrate the charge itorage test arrangement employed, with FIG. 6A showng the equipment arrangement, FIG. 6B showing an :quivalent circuit, and FIG. 6C showing a simplified equivilent circuit.

FIGS. 7A-7E illustrative various charge storage time )btained with the test setup shown in FIG. 6,

FIG. 8 is a graph showing the spectral response of a ihallow diiused n+p junction, and

FIG. 9 is a graph showing the response time of shallow iiifused n+p junctions.

The invention will now be described in detail by ref- :rence to a preferred embodiment thereof which employs iilicon as the target material. FIG. 1 illustrates a vidicon :amera tube 10 of essentially standard construction with he exception of the target 12 which employs the unique esign of a mosaic array of reverse biased junction diodes recording to the present invention. The tube 10 comprises 1n evacuated envelope 14 within which the target 12 is Jositioned at one end so as to be exposed to a radiation mage (indicated by the arrows 15). At the opposite end if the tube 10 is positioned an electron beam forming ind scanning system. Such systems are well-known, from 1o part of the present invention, and need not be described in detail here. Briey, the beam-forming and scanning system is of conventional design and operation and in- :ludes an electron gun assembly 16 and deection means l8 whereby an electron beam can be formed and deiected to scan the target 12 in a predetermined and welltnown manner. lPositioned adjacent the target 12 is an :lectrode mesh 20 for collecting secondary electrons, as ,s well-known in the art. Electrical leads are shown for :onnecting the cathode of the gun assembly 16, the col- ,ecting mesh 20, and the transparent front electrode 22 3f the target 12, to suitable voltage sources 24 and 26 as is well-known.

The target 12, made according to the present invention, ncludes a glass faceplate 32, which in the embodiment ihown in FIG. 1, consists of one end wall of the envelope [4, although it can be a separate element. Positioned in :ontact with the faceplate 32 is a silicon base wafer 28 iaving a transparent film of conductive material coated )n that surface of the silicone wafer 28 which is in contact with the faceplate 32, said film forms the front electrode Z2. The other surface of the silicon wafer 28 is provided with a layer 30 which comprises a mosaic or array of dis- :rete dots or small area islands. The silicon wafer 28 is in n-type single crystal silicon wafer or resistivity be- ,ween 0.01 and 0.1 ohm-cm. An oxide layer is grown )ver one face or surface of the wafer 28 by thermal arocessing in steam. A photo-resist process is used to form 1n array of holes in the oxide layer corresponding to the iesired resolution (e.g. 500 lines/in. for television use). Fh wafer 28 is then placed in a diffusion furnace and a n-type impurity is thermally diffused through the holes n the oxide layer to form an array of discrete n-p juncions. The transparent conductive front electrode 22 is lhen formed on the opposite face or surface of the wafer Z8 through another thermal diffusion (which produces in ohmic iunctionl- After this construction is completed the wafer is ready to mount in the vidicon tube 10 in contact with the glass faceplate 32.

FIG. 2 is a front view of a target according to the invention, such as that shown in FIG. 1, and shows a mosaic array of p-n junctions formed by discrete islands of layer 30 on base wafer 28.

The electron beam coming from the cathode of the gun assembly 16 is accelerated to a few kilovolts at the mesh 20, by the potential applied therebetween from the voltage source 24. This high velocity electron beam will travel through the openings in the mesh 20 and be decelerated toward the potential at the surface of the target 12. In a short time this surface will become charged to the potential of the cathode. A small potential is applied from the voltage source 26 between the target electrode 22 and the cathode of the gun assembly 16. This potential appears across the eleme-nts of the target 12 that are struck by the electron beam, since the charged surface of the target 12 is insulated from the front electrode 22 by the resistance of the body of the target material. If the beam scans every element of the target 12 in some sequential fashion, the whole target 12 will experience this applied potential across its thickness if the time taken to scan the surface is less than the dielectric relaxation time of the material of the target 12. Otherwise, the part of the target 12 that was scanned first would lose its charge before the last part of the target 12 is scanned. Since a typical television scan time is 1/0 of a second, the requirement on the vidicon target material, for successful television operation is that the charge relaxation time be greater than 1&0 of a second, for no incident illumination.

In the operation of known vidicon camera tubes using, e.g. antimony trisulde, a light pattern is focused on the vidicon target through the front transparent electrode and the elements of the target that are struck by light lose their charge since the target material is photoconductive. This occurs because the light-induced electron-hole pairs reduce the resistance of the target body and thereby decrease the RC relaxation time. When the scanning electron beam returns to an element that has been discharged by light during the previous scanning cycle, it quickly charges this element back up to cathode potential. In so doing a current ows in the external circuit (through the resistor 32 of FIG. 1), and it is this current which provides the television signal indicating the presence of light at that particular point on the target. The electron beam charges an element in microseconds, as it continuously moves over all of the screen elements in succession, but each element has 11,0 of a second to be discharged by incoming light. This gives the vidicon a Mg() of a second integration feature which is very important in building up detectable signals at low light levels. Without this integration feature, the vidicon could not compete with the Image Orthicon, which uses secondary emission multiplication to amplify light-induced signals up to one million times.

Referring back now to the present invention and the operation of the vidicon camera tube of FIG. 1, the charge storage mechanism of target 12 is somewhat different from that of known vidicon camera tubes described above. The achievement of charge relaxation times greater `than 1&0 of a second, in relatively low resistivity materials and its application to vidicon targets is the essential feature of this invention. In order to understand the mechanism for this, consider the n-p junction structure of FIG. 1.

In considering such a junction (either p-n or metalsemiconductor) as a target element of a vidicon, it is important to recall that the electron beam succcessively charges each element to a predetermined potential. It is essential to know how long a voltage and its corresponding dielectric charge remain on a reverse biased junction once the source of charge has been open circuited from the element (Corresponding to the electron beam moving on to an adjacent but isolated p-n-junction element). FIG. 3 shows the equivalent circuit for such a reverse biased element, and includes a voltage source 40, a switch 42, a resistor 44, and a capacitor 46 and resistor 48 in parallel. According to semiconductor theory, the reverse biased junction behaves as a parallel plate capacitor Whose plate separation is given by the depletion depth d. 'Ihis depth is a function of applied reverse voltage and various material parameters, chiefly the base layer doping. When an applied reverse bias voltage is removed, the junction acts as if it Were a charged capacitor of plate separation d. The charge does not collapse instantaneously but decays through the junction leakage currents, i.e., reverse saturation current and edge leakage current. For calculation purposes a reverse biased and then open circuited p-n junction capacitor being discharged through its own leakage current was used. Since the reverse saturation current is ideally constant, independent of junction voltage or charge, the general expression dq/dt=iR, integrates out to (q-Qi)=iRt. This relationship is plotted in FIG. 4A. FIG. 4B shows the charge of voltage with time across such a junction, according to the results derived below.

Consider an ideal p-n junction which has been reverse biased to some voltage V1 corresponding to some charge Q1; the junction is then instantaneously open circuited. We determine the decay of charge and voltage across the junction as follows. The change of charge with time is simply given by dq/ dt-iR l wherein iR is a constant, and q=Qi at t=0. Thus (l) integrates to l-Q1=Rt (2) and is plotted in FIG. 3A. The-charge q is related to the applied junction voltage through the capacitance C, which is itself voltage dependent where a represents material constants, and the built-in voltage is assumed small compared with V. Substituting q=CV in (1),

a (M) i dt R (4) Integrating (4) with the initial condition that V=Vi at where (constant) :iR/a

The relation between the initial charge on the junction Qi, and the initially applied voltage Vi is where i refers to initial state. The decay time or current flow time f of this initial total charge is given by where iR is the constant total reverse. leakage current per unit area. From semiconductor theory, for a reverse biased junction,

C1=ee0A/d (30) where d is the depletion depth, A is the area, and e is the dielectric constant. Substituting and (30) in (20),

where Vd is the diifusion potential of the barrier layer, V, is the applied potential, e is the electron charge, and Nd is the donor density of the N type base wafer. Substituting (50) into (40) with the assumption of a stepjunction, `V1 Vd, and Nd=1pepm (junction open-circuited charge decay-time) (bulk photoconductor charge decay time) It is important to note that high resistivity is required for vidicon operation with bulk photoconductors, while low resistivity is necessary for the bulk material used to make junctions. Since it is almost always possible by proper doping to obtain low resistivity from high resistivity material (but not the converse), a wide range of materials may be adoptable to junction structures for use on. vidicon targets. Infrared responsive charge storage vidicons, for example, are not possible with bulk photoconductive targets at room temperature since a small bandgap (i.e., less than 1.1 ev.) is implicit, and the intrinsic rcsistivties of 1011 ohm-cm. required for vidicon operation are not obtainable in materials with bandgaps less than about 1.7 ev. at room temperature.

-Equation 60 shows that the charge storage time in the open circuited junction increases in proportion to the one-` half power of the reciprocal of the base resistivity. However, the base resistivity cannot be chosen to have an indiscriminately low valve to achieve the longest charge storage time. The Zener breakdown electric iield strength sets a lower limit on the useful base resistivity. The maximum electric iield strength Fd=2V1/d, at a step junction interface is related to the applied bias voltage and the base resistivity in the following manner:

The electric field strength, F01 at which Zener breakdown begins must not be exceeded in normal device operation. Since the vidicon target potential, V1, is typically lO v. o1' less, Equation 80 sets a lower limit on the useful base resistivity:

l 2 20 p (FO,) (Esch) (for V,-10 volts) (90) Since the charge decay time for normal vidicon operation Imust be greater than V30 of a second, Equation 60 with 1=Jy0 of a second, sets an upper limit on base resistivity. Equations 60 and 90 together define a range of base resistivity in which junctions can be used as charge storage vidicon targets (basic junction charge storage criterion) Any photosensitive material in which a junction can be obtained which satisfies can be used as a charge storage vidicon target, with a dark state charge storage time of at least lfm of a second. Materials with junctions which cannot satisfy (100) will show dark charge storage times of less than J/go of a second or Zener breakdown.

The parameters in Equation 100 are well lknown for silicon, and this material can be readily evaluated for junction charge storage vidicon operation at room temperature. The Zener breakdown electric eld strength Fui in silicon is approximately 106 v./cm. Since vidicon operation requires a target potential V, up to 10 v., the left iide of (100) sets a lower limit on the base resistivity, 1 1.6 102 ohm-cm. In order to evaluate the right side nf (100), the reverse bias saturation current iR must be cnown. This quantity can be readily calculated from well cnown equations if the doping density and lifetime of the )ase material are known. We have consistently obtained :xperimental values less than 10c@ A./cm.2 in a junction nesa construction, and this value seems a reasonable lpper limit for good planar technique as well. Assuming L reverse saturation current of l A./cm.2, and V1=l0 I., (100) predicts that for silicon junctions to exhibit everse `biased dark state charge storage times greater :han 1/0 of a second, the following criterion must be Jbeyed: l.t3 l02 ohmcm. p 4 ohm-cm. (for charge itorage diode vidicon operation in silicon junctions exlibiting reverse currents of 10 A./cm.2). It is evident, herefore, that state of the art silicon step junctions can Je constructed which satisfy the requirement for charge storage vidicon operation at room temperature. As the -esisti-vity p of the lightly doped n-side approaches 10-2 )hm-cm., the charge storage time increases; all junctions with resistivities below 4 ohm-cm. and reverse leakage :urrent less than 10-6 A./cm.2 will yield at least 1&0 of 1 second charge storage. FIG. 5 plots charge storage time 1s. resistivity p with iR as a parameter, from (60).

The requirements for charge storage vidicon operation it room temperature in germanium junctions are more :tringent because of the greater reverse leakage current. Phe breakdown electric eld strength in germanium is aparoximately l05 v./cm., and the left side of (100) thus iets a lower limit on the base material resistivity for l0 v. arget potential operation, p 1 ohm-cm. The deple- :ion depth for a step junction in germanium constructed rom l0-1 ohm-cm. base material with l0 v. bias is, from (50) approximately lp. The capacitance of such a ievice is thus 1.4 108 f./cm.Z From (20), if the charge iecay time is to be longer than 1/30 of a second, the 'eversc leakage current must be less than The predictions of this theory have been checked with iorne large area experimental diode structures in silicon. l`hese diodes were constructed using mesa diffusion technology, and reverse biased charge storage times were neasured in a pulse setup.

The theory of the previous section was carried out ipecically for p+n structures, since it will be shown in :he following section that electron beam charging procasses require this structure rather than an n+p. The charge iecay processes, however, are the same in both struc- :ures and the theory is changed only by substituting p for n The experimental work, described below, was :arried out on n+p diodes for convenience.

It is noted here that the charge-storing, photo-sensi- ;ive, image transducing structure of this invention need lot always be a p+n structure since the mosaic array surace can be charged positively. In the case in which the nosaic surface is charged positively an n+p structure is ised in order to properly obtain reverse biasing. Such positive charging can be accomplished, eg., by corona :harging or positive ion beam scanning.

Several 10 mil thick wafers of 0.1 ohm-cm. p-type sili- :on were placed in a phosphorous vapor stream at 950 C. for minutes to form a sub-micron thick n+-type surface inversion layer. One side was etched away to aroduce a wafer thickness of 8 mils with a phosphate glass on one face only. The wafers were then placed in 1 boron vapor stream at 900 C. for 15 minutes to form t p+ contact on the previously etched wafer face. A 1 zm.2 mesa was etched in both surfaces to form an n+pp+ structure. Identical operations were performed on 1000 )hm-cm. p-type wafers. We have thus constructed several n+-pp+ step diodes on low resistivity base material (which fullls the theoretical criterion for long charge .'elaxation times), and have constructed several identical TABLE I EXPERIMENTAL DIODE PARAMETERS Saturation Depletion current depth, AJcrn.z

(6 v. (6 v. reverse reverse circuit bias) bias) decay) n+p Phosphor- 30 10' 1 ous d itused mesa. .do 0. 1 2X1()-7 RC, msec. (open Base material Junction type (A) 1,000 p-cm. p-type silicon.

(B) 0.1 p-ern. p- 50 type silicon.

These diodes were tested for reverse biased charge storage. FIG. 7 shows that the RC charge storage time of the 0.01 ohm-cm. silicon base junction is at least 1/30 of a second and that the high resistivity base silicon (1000 ohm-cm.) does not give an RC product which even approaches the 1/90 of a second desired for true vidicon operation.

For successful use as vidicon targets, a silicon junction must show efficient photoconductive properties and fast time responses to changing light levels, as well as charge storage. The spectral response curve for one of the charge storage junctions is given in FIG. 8; the response time to a pulse from a gallium arsenide light emitting diode is shown in FIG. 9. Wide spectral operating range and relatively fast response time characteristics are well known for shallow junctions. However, these data demonstrate that such characteristics can be obtained simultaneously with long charge storage phenomena in low resistivity base material according to the present invention. The quantum eiciency of this device has also been measured. A calibrated thermocouple detector was used to determine the light power incident on the diode from a 25 00 K. tungsten lamp, and the induced photocurrent was measured. A value of 0.30 AJ/IW. was obtained for a quantum eciency of 35% for 1u radiation. Such junctions have also been tested under vacuum conditions, and they show improved reverse leakage current characteristics in vacuum, rather than any degradation.

In order to maintain an image on a vidicon target, it is necessary that the lateral resistance be high enough to isolate individual elements, so that the illuminated elements of the image do not spread. A single silicon junction obviously will not satisfy this requirement since the lateral resistance in all cases is quite low; however, an array of small individual junctions could satisfy the requirement. Isolation would be attained through the reverse biasing of the junctions in a typical vidicon scan operation. In order to attain a 500 line television resolution, individual elements with dimensions of about 0.0015 by 0.0015 in. must be formed, with an interelement separation of 0.0005 in. Since it is not necessary for leads to be attached to the individual elements, the problem is less complex than that for processing monolithic integrated circuitry. These elements can be constructed using photolithography and KPR techniques.

With reference to the target 12 of FIG. 1, the incoming light penetrates the transparent front electrode 22 and forms electron-hole pairs in the n-silicon base wafer. These pairs diffuse to the discrete junction elements on the back silicon surface facing the electron beam, and discharge the associated junction elements. The diifusion of minority carriers from the front to the back surface demands a very thin wafer for two reasons;

(1) some spreading will be associated with the diffusion process, Iand this must be kept to a minimum; (2) the minority carriers must not have to travel so far that they die before reaching the back junction elements. The first of these reasons demands that the wafer thickness be less than the desired resolution, so that resolution is not degraded by the lateral diffusion. This requires a wafer thickness of less than 0.0025 in., which is possible with polishing and etching techniques. The second requirement is that the diffusion length must be greater than the wafer thickness. For one ohm-cm. material, frp=l-6 sec. and Lp=(D1-p)/2 0.002 in. Diffusion length considerations also require wafer thicknesses of less than 2 mils.

It is noted from FIG. 1 that the electron beam scans areas of the silicon target between the junction elements as well as the junction elements themselves. In a typical vidicon electron collection system, these nonjunction areas would contribute a large undesirable signal. There are two means for eliminating a signal from such nonjunction areas: (l) coat the nonjunction areas with an evaporated dielectric layer; (2) use a planar oxide construction technique. The rst remedy requires a somewhat difiicult mask alignment procedure during construction, but can be used with mesa type construction. The second remedy is a natural result of a mosaic array construction process using planar oxide technology. In this case, the n-silicon base Wafer would have a thick oxide formed on one surface, holes with the desired resolution would be photoetched through the oxide, and a p-type dopant would be diffused through the exposed areas. No leakage signal wo-uld appear when the beam struck nonjunction areas, as long as the oxide layer had a resistivity above about 12 ohm-cm.

The discussion of charge storage in reverse biased p-n junctions has been based on either n+p or p+n structures. However, it is important to note that considerations of the electron beam scanning system require an n-type base and a p-type inversion layer in order to operate in the typical vidicon mode (i.e., with the energy of the readout beam less than the first crossover potential with respect to the surface of the photosensitive layer). This is necessary because the electron beam charges the back junction surface to a negative potential; in order to reverse bias the junction, the p-side must become negative. Thus the p-side must face the incoming electron beam, and the n-type base must face the incident light. `In a system using a positively charged beam of particles the base would be p-type and the mosaic array would be of n-type material.

It has been shown above that relatively small bandgap, low resistivity semiconductors (such as silicon and germanium) can be used as charge storage type vidicon targets at or near room temperature by employing specially designed p-n junction mosaic arrays. A rather surprising result of the analysis is that low resistivity doped base material must be employed in the p-n photojunction vidicon, in contrast to the usual requirement for extremely high resistivity in a bulk photoconductor vidicon.

Any semiconductors that are photosensitive to the extent that imagewise exposure by actinic radiation (e.g. light, X-rays, infrared radiation, gamma rays, particle radiation, etc.) of a charged target of such semiconductor material, constructed according to the present invention, will produce a corresponding imagewise change in the surface charge characteristics thereof, which change is sufficient to allow the scanning readout electron beam to distinguish between exposed and nonexposed areas thereof, are useful in the present invention and are herein referred to as photosensitive semiconductors.

The image transducing system of the invention is not limited in application to use in vidicon camera tubes. The conversion of an optical image into an electrostatic image by this invention can be employed to produce a visible print according to known electrographic methods. For

example, the target 12 can be corona charged, imageexposed to produce an electrostatic image and this electrostatic image can be transferred to an insulating sheet where it can be Xerographically developed to produce a print corresponding to said light image. Further, it is noted that the p-layer and the n-layer can be of different semiconductor materials. The mosaic array of p-n junctions can be used as a photosensitive, charge storing grid in which case both layers are essentially formed as mosaic arrays.

What is claimed is:

1. In a vidicon camera tube including an evacuated envelope, a charge-storing photosensitive target adjacent a transparent end of said envelope, which target is adapted for imagewise exposure to actnic radiation, an electron beam forming and scanning means adjacent the other end of said envelope for use in scanning said target, and an electrode mesh mounted adjacent the inside surface of said target for fixing the potential to which said surface is charged by said beam, the improvement wherein:

said target comprises a semiconductive material con? taining a p-n junction between a front n-type layer and a rear p-type layer which rear layer faces said beam and which comprises an array of discrete, substantially uniformly and insulatingly spaced-apart areas of substantially uniform size, said p-n junction having a dark charge-storing time greater than the scan time of said beam,

a front transparent electrode in contact with the front surface of said n-type layer, and

means for maintaining said transparent electrode at a positive potential whereby said p-n junction is reverse biased.

2. The apparatus according to claim 1 in which said semiconductive material is silicon having a resistivity between 0.01 and 0.1 ohm-cm.

3. The apparatus according to claim 1 in which said semiconductive material has a resistivity p in which:

a 2 a 2. (F01) ffon) p (in (2in1 wherein 'F=Zener breakdown electric field strength,

V1=target potential (up to 10 v.),

itn=electron mobility of the n-type layer,

e=dielectric constant of n-type layer,

[f0=dielectric constant of p-type layer,]

iR=reverse bias saturation current.

4. A vidicon camera tube comprising a semiconductor target member having first and second sides, at least the first of which is of n-type conductivity and adapted to receive light images, a plurality of discrete p-type regions in said second side of said semiconductor target member, and electron beam means for scanning said second side of semiconductor target member, said semiconductor target member having a resistivity p wherein:

1 3 2V, 30 2 660V; (El) (eeoun) p (r) -(2th wherein ayer which rear layer faces said beam and which comprises an array of discrete, substantially uniformly and insulatingly spaced-apart areas of substantially equal size, ind including means for reverse biasing said p-n junction, said semiconductive material having a resistivity p wherein:

wherein 'F0=Zener breakdown electric field strength, V1=target potential (up to 10 v.), lin-:electron mobility of the n-type layer, e=dielectric constant of n-type layer, [e=dielectric constant of p-type layer,] iR=reverse bias saturation current.

6. The apparatus according to claim 5 in which said semiconductive material is silicon having a resistivity between 0.01 and 0.1 ohm-cm.

7. The method of producing a vidicon camera tube target element comprising:

providing a first layer comprising an n-type single crystal silicon wafer having a resistivity -between 0.01 and 0.1 ohm-cm., growing an oxide layer over one surface of said wafer, forming an array of holes in said oxide layer, said array corresponding to a resolution of about 500 lines/ inch, l thermally diffusing a p-type impurity through said holes to form an array of discrete p-n junctions, and providing a transparent conductive electrode in contact with the other surface of said wafer.

8. A storage tube comprising:

an evacuated envelope,

an electron beam forming and scanning means adjacent one end of said envelope,

a charge storing, photosensitive target element adjacent the other end of said envelope,

an electrode mesh mounted adjacent the inside surface of said target element for use in fixing the potential to which said inside surface will 4be charged by said beam,

said target element comprising:

a semiconductive material containing a p-n junction between a front n-type layer and a real p-type layer which rear layer faces said beam and which comprises an array of discrete, substantially uniformly and insulatingly spacedapart areas of substantially uniform size,

a front transparent electrode in contact with the front surface of said n-type layer,

means for maintaining said transparent electrode at a positive potential whereby said p-n junction is reverse biased, and

said p-n junction having a dark charge storing time greater than the scan time of said beam.

9. The apparatus according to claim 8 in which said semiconductive material is silicon having a resistivity between 0.01 and 0.1 ohm-cm.

10. The apparatus according to claim 8 in which said semiconductive material has a resistivity p in which:

1 2 2V, :92 2 (eeovi (550MB) P (R) 2lb:

wherein 11. A vidicon camera tube target element comprising: a first layer of photosensitive, semiconductive material wherein:

1 2 3 0 2. eovi (Ful) (ffoun) p (n) (2in1 wherein:

F0=Zener breakdown electric field strength,

V1=target potential (up to 10- v.),

pm=electron mobility of the n-type layer, le=dielectric constant of n-type layer,

[eozdielectric constant of p-type layen] iR=reverse bias saturation current.

12. A vidicon camera tube target element comprising:

a first layer of n-type silicon having a resistivity between 0.01 and 0.1 ohm-cm., and

a second layer of p-type silicon having a resistivity between 0.01 and 0.1 ohm-cm. in contact with one surface of said first layer and comprising an array of equi-spaced, insulatingly separated areas of equal size whereby said target element comprises an array of discrete p-n junctions in silicon.

13. A vidicon camera tube comprising:

(a) a Semiconductor target member having a resistivity between 0.01 and 0.1 ohm-cm. and first and second sides, at least the first of which of n-type `conductivity and adapted for exposure to light images;

(b) a plurality of junction diodes on said second side of said semiconductor target member;

(c) and electron beam means for scanning said second side of said semiconductor target member.

14. The invention according to claim 13 wherein said junction diodes are of the p-n junction type.

15. The invention according to claim 13 wherein said junction diodes are of the metal-semiconductor type.

16. The method of employing a relatively narrow bandgap, relatively low resistivity, photosensitive silicon semiconductor having a resistivity between 0.01 and 0.1 ohm-cm. as a charge-storing image transducing element comprising:

forming a mosaic array of insulatingly separated n-p junctions in said semiconductor,

charging one surface of said array,

reverse biasing said junctions,

exposing said array to actinic radiation, and

reading-out the information stored on said array as a result of said exposing step in a time period less than the dark charge-storing time of said junctions.

17. The method according to claim 16 including employing said array as a vidicon target, and said reading-out step comprising scanning said array with the vidicon readout electron scanning beam having an scan time of about 1/30 of a second, whereby the dark charge-storing time of said junction is greater than said scan time.

18. A vidicon camera tube comprising:

(a) a semiconductor target member having a resistivity between 0.01 and 0.1 ohm-cm. and first and second sides, at least the rst of which is of n-type conductivity and adapted to receive light images;

(b) means forming a plurality of discrete junctions on said second side of said semiconductor target member;

(c) and electron beam means for scanning said second side of said semiconductor target member.

19. The invention according to claim 18 wherein said discrete junctions are of the p-n junction type.

20. The invention according to claim 18 wherein said discrete junctions are of the metal-semiconductor type.

13 21. The invention according to claim 18 wherein said means forming said plurality of discrete junctions comprises a plurality of discrete p-type regions in said semiconductor target member.

22. The invention according to claim 18 including 5 means for reverse biasing said junctions.

References Cited The following references, cited by the Examiner, are

1 4 UNITED STATES PATENTS 3,011,089 11/1961 Reynolds 313-65 X 3,289,024 11/1966 De'Haan et al 313--65 3,322,955 5/1967 Desnignes 250211 X CARL D. QUARFORTH, Primary Examiner P. A. NELSON, Assistant Examiner U.S. C1. X.R.

of record in the patented le of this patent or the original 10 315 12; 313 65 patent. 

